In the field of electrophotographic copying or electrostatic printing, it is a common practice to use, for developing an electrostatic latent image, toner in the form of colored resin particles containing coloring agents and electric charge controlling agents dispersed in a resin medium.
Generally, the toner is composed of resin particles containing coloring agents, electric charge controlling agents and the like as mixed and dispersed in binding resin. To provide the toner with desired color and electrostatic chargeability, it is a common practice to suitably change the types and blending amounts of the binding resin, coloring agents, electric charge controlling agents and the like to be used.
As a toner manufacturing method, there is generally adopted a so-called pulverization method by which a resin medium and coloring agents are molten and kneaded to prepare toner in the form of particles having a predetermined range of particle size. However, the toner thus produced by the pulverization method contains particles having irregular shapes and presents a poor fluidity. Further, the individual irregular particles of the toner are electrically, charged in considerably different manners, causing the distribution of electrostatic charge to become broad. Further, the pulverization method requires facilities of great size, resulting in increased production cost.
It is also proposed to manufacture toner by a suspension polymerization method. According to this method, a polymeric composition in the form of a mixture of an initiator of polymerization with toner characteristic imparting agents such as polymeric monomers, coloring agents, electric charge controlling agents and the like, is suspended, under stirring at a high speed, in an aqueous solution containing a dispersion stabilizer. Then, this mixture is polymerized to directly produce toner. This toner manufacturing method using suspension polymerization may directly produce toner in the form of particles of which sizes are in a practical range, at the resin polymerization step. This results in decrease in production cost. Further, this method presents the advantage that the resultant toner is excellent in fluidity and stability of electrostatic charge.
Such a toner manufacturing method using pulverization or suspension polymerization uses an electric charge controlling agent for adjusting the characteristics of toner electrostatic charge. As the electric charge controlling agent, a variety of dyes are generally used. Since the electric charge controlling agent controls the developing properties of the toner, it is important to properly select the electric charge controlling agent.
More specifically, the demands for an image forming apparatus are recently versatile according to applications and are extended to a variety of performances such as smaller-size, lower-energy, higher-speed, multi-colors, maintenance-free and the like. Accordingly, to accomodate to the developing system or inside environmental conditions of the apparatus, the toner is required to have different characteristics according to the apparatus types and colors used. Therefore, a great number of types of toner are apt to be produced in a small amount. It is therefore required to manufacture, with good reproducibility, toners of which characteristics are subtly different. In view of the foregoing, the selection of the electric charge controlling agent is important.
However, the dyes are hardly compatible with resin and a polymeric monomer. Accordingly, a great amount of dyes should be added to obtain a sufficient electrostatic charge. Further, since the dyes are present in the form of particles in resin and a polymeric monomer, the electrostatic chargeability of the resultant toner considerably vary with the quality of dispersion of the dyes. Such variations may cause image fog, toner scattering and uneveness of an image quality. Further, when the dyes are used for color toner requiring light permeability, the dyes dispersed in the form of particles inhibits such light permeability, failing to form a clear color image. In addition, the dyes are generally expensive, leading to increase in production cost.
In view of the foregoing, to uniformly stabilize the characteristics of toner electrostatic charge, in particular to improve the starting electrostatic chargeability, it is proposed to mix and uniformly disperse binding resin and a copolymer, serving as electric charge controlling resin, composed of (i) a monomer having a polar group and (ii) an oil-soluble monomer compatible with the binding resin or a monomer capable of forming the binding resin, so that the electrostatic charges of individual toner particles are made uniform. For example, Japanese Unexamined Patent Publication No. 88564/1988 discloses toner containing a polymer (copolymer) having a sulfonate group connected to an aromatic ring.
It is found that the sulfonate group disclosed in this Publication is excellent in electric charge imparting properties and that the use thereof for toner improves the electrostatic chargeability such as starting and stabilization of the electrostatic charge, and the like.
However, the inventors of the present invention have found the following facts. That is, even though the monomer having a polar group (sulfonate group or the like) and the oil-soluble monomer in the electric charge controlling resin are substantially the same in monomer composition ratio, the dispersibility of the electric charge controlling resin in the binding resin considerably depends on the polymer structure or molecular weight of the electric charge controlling resin which varies with change in production conditions, difference in raw materials used between lots, and the like. This presents the problems that the electrostatic chargeability undergo a change, that the water vapor resisting properties are lowered, and that the hue varies, failing to repeatedly produce the toner having desired characteristics with good reproducibility.
Consideration is then made on the use of the electric charge controlling resin for manufacturing toner by the suspension polymerization method. The polar group for controlling the electric charge of the electric charge controlling resin is also water-soluble. Accordingly, when the electric charge controlling resin as mixed in a polymeric composition is subjected to suspension polymerization, the electric charge controlling resin is eluted from the suspension oil drops into water. Further, the emulsification by the water-soluble polar group of the electric charge controlling resin thus eluted, causes a by-product in the form of particles with 1 .mu.m or less to be produced. This may not only reduce the productivity, but also deteriorate the electrostatic chargeability, durability and water vapor resistance of the toner.
It is a main object of the present invention to provide electric charge controlling resin excellent in electric charge imparting properties and dispersion in binding resin.
It is another object of the present invention to provide electric charge controlling resin which is adapted not to be eluted in an aqueous phase in a suspension polymerization step while being maintained in suspension oil drops until the polymerization is complete, and which may be uniformly and evenly dispersed in the oil drops.
It is a further object of the present invention to provide a toner producing method capable of manufacturing toner having improved durability with good productivity and without production of a by-product in the form of particules at a suspension polymerization step.
It is still another object of the present invention to provide electric charge controlling resin capable of imparting uniform and highly reproducible electrostatic chargeability.
It is a still further object of the present invention to provide toner having desired characteristics and a method of producing such toner in a system of producing a great number of types of toner in a small amount.
It is yet another object of the present invention to provide toner which may be economically produced with increased productivity, and a method of economically producing such toner with increased productivity.
It is a yet further object of the present invention to provide toner which is excellent in starting and stability of electrostatic charge and light permeability, and which is also excellent in coloring properties and water vapor resistance, and to provide a method of producing such toner.